U.S. Pat. Nos. 4,640,352 and 4,886,118 disclose conductive heating of subterranean formations of low permeability that contain oil to recover oil therefrom. Such low permeability formations include hydrocarbon-bearing diatomite formations. Diatomite is a soft rock that has very high porosity but low permeability. Heat injection methods to recover oil are particularly applicable to diatomite formations because these formations are not amenable to secondary oil recovery methods such as water, steam, and carbon dioxide flooding. Flooding fluids tend to penetrate formations that have low permeabilities, preferentially through fractures. The injected fluids therefore bypass most of the hydrocarbons in the diatomite formations. In contrast, conductive heating does not require fluid transport into the formation. Oil within the formation is therefore not bypassed as in a flooding process.
When the temperature of a formation is increased by conductive heating, vertical temperature profiles will tend to be relatively uniform because formations generally have relatively uniform thermal conductivities and specific heats. Transportation of hydrocarbons in a thermal conduction process is by pressure drive, vaporization, and thermal expansion of oil and water trapped within the pores of the formation rock. Hydrocarbons migrate through small fractures created by the expansion and vaporization of the oil and water.
Considerable effort has been expended to develop electrical resistance heaters suitable for injecting heat into formations having low permeability for thermal conductive heating of such formations. U.S. Pat. Nos. 5,065,818 and 5,060,287 are exemplary of such effort. U.S. Pat. No. 5,065,818 discloses a heater design that is cemented directly into a formation to be heated, eliminating the cost of a casing in the formation. However, a relatively expensive cement such as a high-alumina refractory cement is needed.
Gas-fueled well heaters which are intended to be useful for injection of heat into subterranean formations are disclosed in, for example, U.S. Pat. Nos. 2,902,270, and 3,181,613 and Swedish Patent No. 123,137. The heaters of these patents require conventional placement of casings in the formations to house the heaters. Because the casings and cements required to withstand elevated temperatures are expensive, the initial cost of such heaters is high.
U.S. Pat. No. 5,255,742 (application Ser. No. 896,861 filed Jun. 19, 1992) and U.S. Pat. No. 5,297,626 (application Ser. No. 896,864 filed Jun. 19, 1992,) disclose fuel gas-fired subterranean heaters. The heaters of these patents utilize flameless combustion to eliminate hot spots and reduce the cost of the heater, but still use high alumina refractory cements to set the burner within the formation.
When heaters are cemented into a formation for the purpose of heating the formation, the cement must be uniform. It is very difficult to cement such an apparatus into a wellbore without leaving some voids around the heater. The voids will prevent uniform heat transfer from the wellbore, and could result in premature failure of the heaters in the vicinity of the voids. Further, voids in cement result in unsupported portions of the heater. These unsupported portions are weak points that will fail if the burners are subjected to stress by, for example, subsidence due to withdrawal of hydrocarbons from the surrounding formation. Cement for heat injection wells, preferably, has high thermal conductivity. High alumina cements have such high conductivity but are expensive and high alumina cement slurries have very high densities. High density slurries can fracture diatomite formations.
It is therefore an object of the present invention to provide a method and apparatus to inject heat into a subterranean diatomite formation using a heater that does not require expensive cements or casing materials. It is a further object of the invention to provide a method and apparatus wherein a smaller wellbore is possible, thus reducing drilling costs. Another object of the invention is to provide a method and apparatus wherein the sintered diatomite results in a stable high temperature sheath around the well.